1. Field of the Invention
The present invention relates to a vibration eliminating apparatus and particularly to a vibration eliminating apparatus for eliminating any vibration from an installation floor by suspending by means of a magnetic force a vibration-free table on which equipment is installed to be isolated from vibration.
2. Description of the Related Art
Equipment such as electromicroscopes and semiconductor manufacturing apparatuses or the like which must be isolated from vibration is installed on a vibration eliminating apparatus in factories. Instead of a conventional vibration eliminating apparatus utilizing pneumatic springs or rubber, a vibration eliminating apparatus utilizing magnetic suspension means has been developed to realize excellent vibration suppressing characteristics, as disclosed in detail in Japanese Patent Laid-Open No. 203040/92.
FIG. 1 schematically illustrates an example of the structure of a basic vibration eliminating apparatus using a magnetic suspending system. Equipment such as electromicroscopes, semiconductor manufacturing apparatuses or the like (not illustrated) which must be isolated from vibration is mounted on a vibration-free table 1. The table is maintained without any contact with an installation floor (not shown) by means of electromagnetic actuators 2. Therefore, even if the installation floor on which the electromagnetic actuators 2 are installed vibrates due to earthquakes or any other reason, no vibration is transmitted to the vibration-free table 1 and therefore the equipment on the table 1 is not affected by vibration of the installation floor.
FIG. 2 shows the structure of the electromagnetic actuator used in the vibration eliminating apparatus shown in FIG. 1. Yokes 3 consisting of a magnetic material are fixed to the vibration-free table 1 on which equipment (not shown) which must be isolated from vibration is installed. Electromagnets 4 fixed on an installation floor 6 suspend the magnetic yokes 3 by means of magnetic attracting forces. A gap between a magnetic pole of the electromagnet 4 and the magnetic yoke 3 can be measured with a displacement sensor 5. A controller 7 includes compensation circuits 9 and power amplifiers 10 whereby an excitation current flowing through electromagnet 4 is controlled in accordance with an output from the displacement sensor 5 so that the vibration-free table 1 to which the magnetic yokes 3 are fixed may be stably suspended.
However, the vibration eliminating apparatus using magnetic suspending forces as explained above is disadvantageous in that a heavy current flows continuously through the excitation coils of the electromagnets, resulting in the generation of excessive heat from the excitation coils and a large power loss, because the electromagnets must always be excited to suspend the vibration free table.